Abhesive coating compositions are used on a large scale for coating the surfaces of a variety of materials, especially flat or plane materials, to reduce the adhesive tendency of adhering products towards these surfaces. Abhesive coating compositions are used, for example, to coat papers or films which are to be used as backing or carrier for pressure-sensitive labels. Labels, which are provided with a pressure-sensitive adhesive, still adheres to the coated surface to a sufficient extent to enable the backing films with the adhesive labels thereon to be handled. However, it is a prerequisite to be able to peel off the labels from the coated backing film without significantly reducing their adhesive force for later use. Further applications for abhesive coating compositions are packing or wrapping papers which are used to wrap sticky materials in particular. Such abhesive papers or films are used, for example, to package or wrap foods or industrial products, such as bitumen.
Cross-linkable organopolysiloxanes, especially radiation-curable organopolysiloxanes which are modified with acrylate ester groups have proven to be suitable as abhesive coating compositions.
For example, a method for preparing organopolysiloxanes, modified with pentaerythritol triacrylate or pentaerythritol methacrylate esters, from organochloropolysiloxanes, optionally with HCl-binding neutralizing agents, is known from German Patent No. 29 48 708. For this method, organopolysiloxanes of the formula ##STR2## (R.sup.1 =alkyl with 1 to 4 carbon atoms, vinyl and/or phenyl, with the proviso that at least 90 mole percent of the R.sup.1 groups are methyl; a=1.8 to 2.2; b-0.004 to 0.5) are first reacted with, based on the SiCl groups, at least 2 moles of a dialkylamine. The alkyl groups of this dialkylamine each have 3 to 5 carbon atoms and the carbon atoms, adjacent to the nitrogen, carry no more than one hydrogen atom. The reaction product is then allowed to react with at least equimolar amounts of pentaerythritol acrylate or methacrylate and finally separated from the solid components suspended in it by a known method.
A coating composition prepared according to this method shows relatively good abhesive properties. Adhesive tape, in contact with the coating composition, largely retains its adhesive force relative to untreated substrates. However, it has been ascertained that the property of abhesiveness must be considered in conjunction with the chemical composition and structure of the adhesive, relative to which the coating material is to have abhesive properties. The coating material described in German Patent No. 29 48 708 therefore cannot provide satisfactory results in all cases, since its properties cannot be matched to different adhesives.
Improved properties were exhibited by (meth)acrylate ester-modified organopolysiloxane mixtures which are characterized in that they consist of an equilibrated organopolysiloxane with, on average, &gt;25 to &lt;200 silicon atoms, 2 to 30% by weight or organopolysiloxanes with, on average, 2 to 25 silicon atoms and 2 to 30% by weight of organopolysiloxanes with, on average, 200 to 2,000 silicon atoms. Such compounds are disclosed in German Patent No. 34 26 087. Different tasks are assigned to the organopolysiloxanes contained in this ternary mixture. The low molecular weight fraction essentially must fulfill the task of ensuring the adhesion of the coating composition to the substrate. The high molecular weight fraction serves primarilly the purpose of achieving the desired abhesiveness of the coating composition. The middle fraction is the curable matrix which is responsible especially for the physical properties of the coating composition. Those skilled in the art will understand that this is merely a simplified description of the properties and tasks of the three different fractions since the complex properties which an abhesive coating material must have can be obtained only by the cooperation of the three components. It has become possible with the modified organopolysiloxane mixture of German Patent 34 26 087 to improve, on the one hand, the abhesive properties of the mixture relative to adhesive surfaces and, on the other, the adhesive properties relative to the substrate on which the coating material is applied and on which it is cured. However, it has been ascertained that the properties of even these coating compositions cannot be adapted adequately to different adhesives.
In European Offenlegungsschrift 0 159 683, electron beam-curable liquid coating materials are described. These contain the following:
1. 60 to 95 parts of an organopolysiloxane with more than about 25 siloxane groups per molecule and 2 to 10 parts of reacted carbinol groups per molecule, the remaining substituents on the silicon being hydrocarbon groups with 1 to 20 carbon atoms; reacted carbinol groups in this context are understood to be esters of acrylic acid, methacrylic acid or mixtures or ethers of a hydroxyalkyl ester of these acids, the alkyl group containing 2 to 4 carbon atoms; unreacted carbinol groups are essentially no longer to be present so that the hydroxyl number is less than 10;
2. 3 to 25 parts of a polyester of a polyhydric alcohol with acrylic acid, methacrylic acid or mixtures of these acids, the polyhydric alocohol having 2 to 4 hydroxyl groups per molecule and a molecular weight of less than 1,200;
3. 1 to 10 parts of acrylic acid, methacrylic acid or mixtures of these acids.
The additional use of the (meth)acrylate ester of a polyalcohol increases the curing rate; however, as a result of the increase in the organic portion, it has a very disadvantageous effect on the flexibility and abhesiveness of the coating material. The content of free acrylic acid or methacrylic acid is a further disadvantage, as it leads to an objectionable odor and makes the processing more difficult when applying the product on the material to be coated.
Because they can be cured by radiation, organopolysiloxanes with acrylate ester groups have been described for a number of additional applications. Organopolysiloxanes, modified with acrylate ester groups, are thus used as coating lacquers for casting or recasting electric and electronic components as well as for producing molded objects. The following Offenlegungsschriften, Auslegeschriften and patents are referred to for possible structures of such polysiloxanes which are modified with acrylate ester groups.
German Auslegeschrift 23 35 118 relates to optionally substituted acrylate group-containing organopolysiloxanes of the general formula ##STR3## (R is hydrogen or a monovalent hydrocarbon group with 1 to 12 carbon atoms; R' are monovalent, optionally halogenated hydrocarbon groups or cyanoalkyl groups with 1 to 8 carbon atoms; R" are divalent hydrocarbon groups with 1 to 18 carbon atoms or divalent hydrocarbon groups with C--O--C bonds; R'"=R""O.sub.0.5 or R.sub.3 'SiO.sub.0.5 ; Z=OR"", R"" or OSiR.sub.3 '; R"" is an alkyl group with 1 to 12 carbon atoms; a and b each are numbers from 1 to 20,000; c is a number from 0 to 3; e is a number from 0 to 2; at least one of the Z groups is OR"" when c=0.) The siloxane polymers can be used as intermediates in the synthesis of copolymers, which contain organopolysiloxane segments and find application as coating compositions. Moreover, these acrylate-functional siloxane polymers can serve as sizing materials and as protective coating compositions for paper and fabrics. These products, however, are unsuitable for the preparation of abhesive coating materials. The linear diacrylate-modified polysiloxanes of the German Auslegeschrift 23 35 118, moreover, have groups which are defined as alkoxy groups and which can be split off hydrolytically and lead to further cross-linking of the polysiloxanes with deterioration of the elastic properties that are important for a coating composition.
From German Offenlegungsschrift 30 44 237, polysiloxanes with lateral acrylate ester groups are known which can be synthesized by reacting epoxy-functional siloxanes of a particular structure with acrylic acid. The products obtained are radiation-curable. They can be used as low viscosity lacquers for application by way of printing inks based on oil. The products can be used only with major limitations as abhesive coating composition, since each acrylate ester group is facing a hydroxyl group.
In U.S. Pat. No. 4,568,566, curable silicone preparations are described which comprise
(a) 75 to 100 mole percent of chemically bound siloxy units of the formula R.sub.3 SiO.sub.0.5, RSiO.sub.1.5 and SiO.sub.2 and
(b) 0 to 25 mole percent of R.sub.2 SiO units, a number of the R units having the formula ##STR4##
R.sup.1 being hydrogen or a hydrocarbon group with 1 to 12 carbon atoms and R.sup.2 a divalent hydrocarbon group or an oxyalkylene group. These curable preparations are used especially to coat electronic components and as coating materials for optical fibers. They are not suitable as abhesive coating materials for flat backings because the content of R.sub.2 SiO units is too low.
The object of European Offenlegungsschrift 0 152 179 is a silicone preparation which can be cured to form an elastomer. It comprises (a) a silicone resin with linear construction, and on the average, at least 150 siloxane units, as well as terminal acrylic acid groups, the region in between being free of acrylic acid groups, (b) at least 10% of finely divided silica and (c) a photoinitiator. These materials are to be used as adhesives and casting compounds.
Finally, reference is made to European Offenlegungsschrift 0 169 592. This Offenlegungsschrift relates to an optical glass fiber with a plastic coating, with a glass fiber and an enveloping layer of a synthetic rubber with a refractive index which is larger than that of the outer layer of the glass fiber. The synthetic rubber is formed from a curable plastic composition which has a copolymer, the monomer units of which comprise dimethylsiloxane and at least one siloxane of the group formed by methylphenylsiloxane and diphenylsiloxane. The siloxane copolymer has at least two acrylate ester groups per molecule, with the distinguishing feature that the curable plastic composition, moreover, has a polyurethane acrylate with an average molecular weight of more than 3,000. The polysiloxane named in the claim may have the following formula: ##STR5##
It is an essential condition that these polysiloxanes have phenyl groups which are linked to silicon. The phenyl group content is necessary for matching the refractive index of the coating composition to that of the glass of the optical fiber. It is not possible to draw a conclusion from the use of these siloxanes in combination with a polyurethane acrylate for coating optical glass fibers with regard to the possible usefulness of such compounds as abhesive coating compositions.
In the journal, "Makromolekulare Chemie" (Rapid Communication), 7, (1986), 703 to 707, the synthesis of linear methylpolysiloxanes with terminal methacrylate ester groups is described. For this synthesis, an addition reaction between an .alpha.,.omega.-hydrogendimethylsiloxane and allyl epoxypropyl ether is carried out first in the presence of chloroplatinic acid. The diepoxide formed is subsequently reacted with methacrylic acid in the presence of chromium diisopropyl salicylate to the desired methacrylate esters. These esters may be present in two isomeric forms: ##STR6##
Insofar as (meth)acrylate esters are synthesized by the method of the state of the art starting out from epoxy-functional siloxanes, the epoxy groups are reacted with (meth)acrylic acid. In this reaction, monoesters of (meth)acrylic acid with a vicinal hydroxyl group are formed through opening of the epoxide ring.